Bacitracin & Optochin Susceptibility

The bacitracin and optochin susceptibility tests identify whether an organism is susceptible or resistant to optochin and bacitracin. A blood agar plate is streaked with the organism to be tested, and paper discs that have been infused with either bacitracin (A disc) or optochin (P disc) are applied to the surface of the agar. The plate is then incubated. 

Bacitracin is a true antibiotic in that it is an antimicrobial compound which is naturally produced by a microorganism. It is produced by Bacillus licheniformis and acts to interrupt the formation of the bacterial cell wall.  It is not effective on bacteria that do not have cell walls and are not actively growing.  

Optochin is also known as ethylhydrocupreine; it is a derivative of quinine that inhibits pneumococci but does not affect other α-hemolytic streptococci.

If the organism grows up to the edge of the disc, it is resistant to the antimicrobial compound infusing the disk. If there is a zone around the edge of the disk where the organism has not grown, which is called a zone of inhibition, the organism is susceptible to the antimicrobial in the disc. 

The bacitracin test is useful for differentiating β-hemolytic Group A streptococci from β-hemolytic non-Group A streptococci. This is important because most streptococcal diseases are caused by Group A streptococci. The bacitracin test can also be used to differentiate the bacitracin-resistant Staphylococcus from the bacitracin-susceptible Micrococcus. 

The optochin test differentiates Streptococcus pneumoniae from other viridans Streptococci, which are five groups of Streptococci that also demonstrate ɑ-hemolysis.